Certain postage meters include a mechanical counting mechanism for accounting for postage franked. Such counting mechanisms may comprise an ascending register which keeps a tally of the total postage franked by the meter and a descending register which keeps a tally of the amount of credit remaining. When the credit runs low, the postage meter user takes the meter to the postal authorities and buys more credit which is entered into the meter by adjusting the descending register.
Security considerations require that the meter be rendered inoperative when the descending register reaches a pre-set low value (a credit lock-off), thus compelling the user to have the meter recharged. Also, the machine is required to be rendered inoperative after a given accumulated amount of value has been franked (a total lock-off). It is desirable that the credit lock-off may be released merely by recharging the meter with credit, but that total lock-off be releasable only by major disassembly of a meter. This is so that the machine can be examined at a service centre for signs of wear and/or tampering. It is also desirable that the lock-offs be achieved by mechanical means.